Accounting for the aircraft emissions of China’s domestic routes during 2014–2019

The “13th Five-Year Plan” of civil aviation energy conservation and emission reduction impacts China’s domestic aviation exchanges. However, few researchers pay attrition to the impact of the 13th Five-Year Plan on aviation emissions. This paper intends to calculate the emissions of six pollutants (CO2, CO, HC, NOx, PM2.5, and SO2) in China’s domestic airlines from 2014 to 2019 and explore the impact of the 13th Five-Year Plan on emissions changes. In this paper, the improved BFFM2-FOA-FPM method is used to unify the calculation of CO2 and other aviation emissions. The error rate between the estimate and the official data was about 6.45%. The results show that the 13th Five-Year Plan has impacted on aviation emissions, including the number of routes and airlines, aircraft configuration, air routes, and airline unit turnover emissions. Additionally, the 13th Five-Year Plan’s effect is not significant, and it does not promote the reduction of emissions on domestic routes.


Introduction
The global demand for air transportation has grown in the past ten years, and China has become the second-largest air transportation market. The rapid growth of air traffic will bring some economic benefits, but it also impacts the environment1. To protect the environment and reduce the emission of aviation pollutants, the 13th Five-Year Plan for Civil Aviation Energy Conservation and Emission Reduction (13th Five-Year Plan) is proposed to promote aviation emission reduction. The overall goal of the 13th Five-Year Plan is that by 2020, the average energy consumption of unit transport turnover and carbon dioxide emission of civil aviation will drop by more than 4% over the five years compared with the 12th Five-Year Plan. In addition, the average energy consumption of unit passenger throughput of industrial transport airports will drop by more than 15% over the five years compared with the end of the 12th Five-Year Plan, and the garbage will be harmless. On the other hand, the sewage treatment rate of new airports will reach more than 90%. Previous scholars paid little attention to policies when studying the influencing factors of aviation carbon emissions. Considering the impact of the 13th Five-Year Plan on aviation emissions is an interesting part of this paper.
Aviation emissions mainly include carbon dioxide (CO 2 ), carbon monoxide (CO), nitrogen oxide (NOx), sulfur dioxide (SO 2 ), hydrocarbons (HC), and particulate matter (PM2.5) 1,2 . Reducing the emission of pollutants from aircraft has received widespread attention. In 2019, the International Civil Aviation Organization (ICAO) issued emission reduction targets, including reducing greenhouse gas emissions by 50 percent by 2050. The Intergovernmental Panel on Climate Change (IPCC) points out that carbon dioxide produced by human activities is the most significant contributor to the greenhouse effect 3 . The International Civil Aviation Organization (ICAO) uses standards and recommendations from various aspects of international civil aviation to calculate carbon emissions and uses a distance-based calculation method to estimate individual aviation emissions using existing data of different aircraft types 4 .
Some scholars separately measured the carbon emissions at the Landing and Take-Off (LTO) cycle or the Climbing/Cruising/Descending (CCD) stage in the existing studies. In contrast, others estimated the carbon emissions throughout the whole process. LTO phase carbon emissions are generally calculated using the International Civil Aviation Organization (ICAO) standard emission calculation model [5][6][7][8][9][10] . For the CCD emissions, Cui  Other pollutants emitted by aviation mainly include CO, NO x , SO 2 , HC, and PM2.5. Many methods can calculate aviation pollutant emissions, including ICAO advanced method 8,11,16,17 , ICAO Complex method 18-20 , FOA3.0 method specially provided by ICAO for calculating particulate emission index 6,21 , American EPA method 5,22 , and European EMEP method 23-26 , etc. However, the current methods separate calculations of CO 2 emissions from five other emissions. In this paper, the improved BFFM2-FOA-FPM method can unify the analysis of carbon dioxide and other aircraft emissions in the CCD phase, improving existing research. Compared with other methods, the calculation method in this paper unifies the calculation of six emissions, which saves a lot of time and cost. Besides, few current studies have segmented emissions intensity calculations for the voyage. In this paper, the total distance is divided into eight sections every 500 km, and the emission intensity of six pollutants at different distances is calculated. Finally, existing studies rarely consider sub-series of a given aircraft. Therefore, this paper finds specific aircraft types. For example, A320 -232, A320-214, and A320-216 are the sub-series of the A320 series.
Through the research roadmap, we can quickly understand the research ideas of this paper. Fig. 1 is the research roadmap of this paper. This method is also applicable to other countries since the research steps can be copied. Of course, it is customary to encounter some difficulties in research. On the one hand, it is challenging to collect flight information. On the other hand, the error in different years may be substantial. We can reduce errors and improve the reliability of research results by extending the time axis.

Results
This paper has not considered 2020 and 2021 because the pandemic began to spread globally in 2020. In addition, the outbreak of COVID-19 in December 2019 has had a significant impact on China's aviation industry. As a result, China's domestic routes completed a total transportation turnover of 58.767 billion ton-kilometers in 2020. Compared with 2019, China's total domestic transportation turnover in 2020 decreased by 29.2%. Consequently, 2020 and 2021 are not selected in this paper. Therefore, the year from 2017 to 2019 was estab-   The emission intensity of the aircraft in the CCD stage. As mentioned earlier, different from the method of ICAO, we segment each route according to a section of 500 kilometers. Therefore, all routes are divided into 8 distance sections: 0-500 km, 501-1000 km, 1001-1500 km, 1501-2000km, 2001-2500km, 2501-3000 km, 3501-3500 km and 3501-4000 km. In addition, we also consider the differences between sub-series, such as 737-700 and 737-800. Then, we get the aircraft's emission intensity of the six pollutions from 2014 to 2019 based on the Modified Fuel Percentage Method (MFPM). In the 0-4000 km section, 320-214, 320-232, 737-700 and 737-800 cover almost all distances sections. The 320-214 and 320-232 are sub-series of the A320 series, while  www.nature.com/scientificdata www.nature.com/scientificdata/ the 737-700 and 737-800 are sub-series of the B737 series. Therefore, this comparison highlights the difference between this study and the ICAO method.
We summarize the average carbon emission intensity and show the detailed results in Fig. 4. As shown in Fig. 4(a), under the A320 series, carbon emission intensities of 320-214 and 320-232 are similar in the 0-4000 km distance segment. In the 3500-4000 km distance segment, the carbon emission intensity of 320-214 is lower than that of 320-232. However, at other distances, the carbon emission intensity of 320-232 is always less than 320-214. Therefore, 320-232 performs better in carbon emissions per kilometer, providing more references for airlines to arrange aircraft types. Under the B737 series, the carbon emission intensity of 737-700 and 737-800 is similar in 0-4000 km distance, and the 737-700 has always been less carbon-intensive than the 737-800. In addition, with the increase in aviation mileage, the emission intensity is gradually decreasing. This shows that the average emission of long-distance flights is less than that of short-range flights. Therefore, the 737-700 is superior to the 737-800 in the 0-4000 km distance segment. As shown in Fig. 4(b), compared to the A320 series, the B737 series has a lower carbon intensity. Hence, the overall performance of the B737 series at 0-4000 km is better than that of the A320 series.
The impacts of the 13th Five-Year Plan on the overall emissions. First, we check the accuracy of the calculation results in this paper. Then, we obtain each airline's available tonnage and normal load rate from the "Civil Aviation from Statistics" released by CAAC (Civil Aviation Administration of China). The turnover in Table 1 is the total turnover of China's domestic routes, and the fuel consumption per ton kilometer is the common data of China's domestic routes and China's foreign routes. The turnover of each route is the product of available tonnage, normal load rate, and distance. Then we can add up the turnover of all routes. In 2015, the fuel consumption of turnover was about 0.294 kg/ ton. Because of the error, the fuel consumption of turnover can be adjusted by 5%, and the result after adjustment is 0.3076 kg/ t-km. Based on this standard multiplied by the carbon emission coefficient of fuel consumption (3.157 kg/tons), it can be concluded that the carbon dioxide emission of major domestic routes in 2015 was 37,513,993.70 tons. Therefore, the carbon dioxide emissions calculated in this paper are 39,905,708.71 tons, with an error rate of 6.37%. Based on the above calculation method, the calculation errors from 2016 to 2019 are 0.75%, 0.9%, 15.05%, and 9.2%. Considering the statistical data of various airlines may also have errors, the calculation results of the method used in this paper are very accurate.
As mentioned earlier, the main emissions include CO 2 , CO, HC, NOx, PM2.5, and SO 2 , among which CO 2 emissions are much higher than other emissions. For example, in 2019, CO 2 emissions is 56,059,114.91 tons,    Fig. 5, after the 13th Five-Year Plan, the six emissions increased, but the growth rate was less than 35%. Further research finds that the growth rate of NOx is the highest, reaching 34.5%. HC has the lowest growth rate at 23.7%. In terms of the annual growth rates of the six emissions, the growth rates of 2015 and 2016 are higher than those of 2018 and 2019, which may be due to the 13th Five-Year Plan that urges airlines to take some measures to reduce the growth rate of aviation emissions.
In addition, we compared the change in unit turnover emissions of 6 pollutants before and after the 13th Five-Year Plan. According to the calculation of relevant report data from the CAAC, the total annual transport turnover from 2014 to 2019 was 34.88 billion ton-kilometers, 38.62 billion ton-kilometers, and 43.36 billion ton-kilometers, 48.16 billion ton-kilometers, 46.58 billion ton-kilometers, and 54.34 billion ton-kilometers. And then, unit turnover emissions of six gases from 2014 to 2019 can be calculated. The average results of the first three years and the last three years are shown in Fig. 6. The unit turnover emissions of HC decreased, while the unit turnover emissions of the other five gases all increased. Therefore, the 13th Five-Year Plan produces a miniature effect, and China's domestic airline emission reduction work still needs improvement.
The impacts of the 13th Five-Year Plan on the emissions of the routes. We firstly analyze the effect of the 13th Five-Year Plan on the average emissions of airlines. Compared with 2014-2016, the average CO2, NOx, PM2.5, and SO2 in 2017-2019 increased by 2.58%, 4.67%, 1.61%, and 3.45%. In addition, the average emissions of CO and HC decreased by 0.03% and 3.57%. As a result, the average CO2, NOx, PM2.5, and SO2 increased by less than 5%, while the average emissions of CO and HC decreased. As a result, the average emissions growth rate of the six air routes in 2018 and 2019 was negative. Therefore, the 13th Five-Year Plan has slowed down the increase rate of airline emissions.
To investigate the influence of the 13th Five-Year Plan on air routes, we select 251 airlines from 2014 to 2019. However, we split the route data into two groups: 2014-2016 and 2017-2019. Besides, the paper calculates the average unit turnover carbon emissions of two data groups for 251 airlines. The calculation result shows that the average unit turnover carbon emissions of 126 airlines are reduced.
We summarize the impact of the 13th Five-Year Plan on unit turnover emissions of some popular air routes and show the detailed result in Fig. 7. As shown in Fig. 7, this paper picks out the five airlines with the most significant increase and the five airlines with the largest decrease in unit turnover emissions under the influence of the 13th Five-Year Plan. Specifically speaking, the five routes with the most significant increase in unit turnover carbon emissions are Korla-Urumqi, Dalian-Qingdao, Kunming-Lijiang, Shanghai-Wenzhou, and Xishuangbanna-Lijiang. As a result, the average unit turnover carbon emissions of these five airlines increased by 2.  To analyze the impacts of the 13th Five-Year Plan on airlines, this paper selected 27 standard airlines from 2014 to 2019. In addition, airline data is divided into two groups, one is 2014-2016, and the other is 2017-2019. And then, the paper calculates the average unit turnover carbon emissions of two data sets from 251 airlines. The calculation result shows that the carbon emissions per unit turnover of 16 airlines have been reduced. Finally, we summarize the change in carbon emissions per unit turnover of popular airlines before and after the 13th Five-Year Plan in Fig. 8. As shown in Fig. 8, the five airlines with the most significant increase in carbon emissions unit turnover were Grand China Airlines, Donghai Airlines, Qingdao Airlines, Okay Airways,  The main contribution of this paper to the literature is reflected in the following aspects.
In the first place, it is the attempt to combine calculations of carbon dioxide emissions with other aircraft emissions. Some papers focus on calculating CO 2 and other aircraft emissions, but there is no uniform way to link the two accounting methods. In addition, the calculation method of CO 2 emission does not consider the difference of sub-series, and the calculation method of other aircraft emissions also does not pay attention to the emission of the CCD stage. Therefore, we establish a new Modified BFFM2-FOA-FPM method to calculate CO 2 and other aircraft emissions in the CCD stage. LTO emissions are calculated according to the International Civil Aviation Organization (ICAO). The error rate between the computed results and the official data is about 6.45%. This paper applies this method to domestic airlines in China, but it can also be used in other countries or regions.
Secondly, the emission intensity of aircraft types has been calculated first. To ensure the accuracy of the calculation results, we divided the air route distances into eight groups to obtain the emission intensity of 6 pollutants of aircraft types at different distances. As a result, our results cover more detailed aircraft types (such as 320-214, 320-232, and 320-216 under the A320 series) than existing studies, making them more realistic.
The main conclusions and policy recommendations are as follows. First, this paper analyzes the impact of the 13th Five-Year Plan on aircraft configuration. There is no change in the top three (737-800, 320-214, and 320-232) from 2014 to 2019. However, the fourth and fifth places adjusted over time, and 737-700, 321-231, 321-211, and www.nature.com/scientificdata www.nature.com/scientificdata/ 321-213 appeared in the fourth or fifth places. Second, this paper compares the emission intensity of 6 kinds of aviation emissions in the CCD stage. We consider more detailed aircraft types, which is one of the contributions of this paper. We compared the sub-series of the B737 series with those of the A320 series. We found the aircraft types with the lowest emission intensity of the B737 series and the A320 series sub-series, providing a reference for airlines to adjust aircraft configuration. Third, we discover CO 2 always accounts for more than 98% of the total emissions, while the emissions of the other five gases are relatively small. Except for the decrease in HC unit turnover, the additional five gas unit turnover shows an upward trend, indicating that the 13th Five-Year Plan has little impact on domestic aviation. Finally, there were 251 same air routes and 27 same airlines from 2014 to 2019. Under the influence of the 13th Five-Year Plan, 127 air routes and 16 airlines have reduced unit turnover carbon emissions, which shows that the 13th Five-Year Plan impacts the emissions of domestic flights, but this impact is not particularly obvious.
There are technical approaches, market approaches, and management approaches to control aircraft emissions. The technical method mainly involves improving the engine, increasing its fuel efficiency, and using sustainable aviation fuel. The market approaches mainly include aviation pollution tax and carbon emission trading. Calculating carbon dioxide and other aviation emissions is the basis and premise of imposing pollution tax and trading carbon emission rights. Scientific management is mainly reflected in airlines to strengthen the efficiency of air traffic control operation organization and guarantee ability. As mentioned above, the emission intensity of aircraft in different flight segments is different, and the emission intensity decreases with the increase in flight distance. Therefore, airlines can optimize route structure and develop long-distance routes. Thus, the work of this paper is of great significance.
Due to the availability of the data, the specific data analysis in this paper is based on major domestic airlines in China, which is incomplete. Additionally, the factors considered in this paper are not comprehensive enough, such as the age of the aircraft, navigation technology, and aircraft delay caused by various factors, which may have an impact on the emission intensity of the aircraft. Therefore, we can invest more time and energy to obtain more complete data and expand the research scope in future studies.   . dis is the cruising distance, v is the cruising speed, c cr is the fuel consumption ratio when the aircraft is cruising, LD cr is the lift-drag ratio when the aircraft is cruising. The value of c cr and LD cr has direct relationships with the aircraft type. We define ratio cr c LDc cr cr =

Generally
, and then for the cruising task section, the W W / The actual flying time of each flight is applied to check the results of ratio cr , and get the emission intensity. For CO 2 , the emission coefficient is fixed, which is I k g kg 3 157 / CO2 = .
; For SO 2 , the emission coefficient is fixed, which is = . . I j0 is the standard emission coefficient of a LTO stage of NO x (g/kg). θ is the ratio of outside temperature to 288 K; δ is ratio of external pressure to sea level pressure. ϕ is atmospheric relative humidity; P is external pressure; Pv is atmospheric saturation pressure, which is calculated by Goff-Gratch formula 29 According to relevant physical laws [29][30][31]  . ICAO standard method to calculate lTO emissions. The take-Off and Landing stage (LTO) refer to the aircraft's whole process during takeoff and landing. This stage defined by ICAO includes four states, including approaching, taxiing, taking-off, and climbing, which defines climbing as the boundary layer from the end of aircraft takeoff to the aircraft's flight out of the atmosphere. Therefore, this paper uses the standard LTO cycle definition specified by ICAO to calculate the fuel consumption, including all activities at an altitude below 3000 feet (915 m) near the airport. Therefore, this stage is not directly related to the route. In addition, the climbing process requires higher fuel consumption than the cruise phase at a constant altitude. Thus, the Climb, Cruise, and Descent cycle (CCD) are defined as all activities that occur at the height of 3000 feet (915 meters). Thus, fuel use accounts for most of the whole voyage and is directly related to the flight distance. The calculation formula of the five non-CO 2 pollution emissions in LTO stage is: LTO m a a m m E LTO is the emissions in the LTO stage; P a is the standard emissions of the engine of aircraft type a (unit: kg); N a is the number of engines of aircraft type a; C m is the thrust setting of stage m; t m is the working time of phase m. The value range of m is 1, 2, 3, and 4, respectively corresponding to the four stages of takeoff and landing in the aircraft flight process: takeoff, climb, approach and taxiing. According to the standard LTO cycles defined by ICAO, when the aircraft is taking off, its engines are at 100% thrust and working time is 0.7 minutes; when the aircraft is climbing, its engines are at 85% thrust and working time is 2.2 minutes; when the aircraft is approaching, its engines are at 30% thrust and working time is 4 minutes; when the aircraft is taxiing, its engines are at 7% thrust and working time is 26 minutes. Therefore, in a standard LTO cycle, the total working time is 32.9 minutes.
The fuel consumption rate is calculated as: A is the total number of airlines with aircraft type a; j is the type of engine of the aircraft; K j is the number of aircraft type a equipped with engine type j; F jmi is the fuel consumption rate of engine type j under the m setting. The data is from the ICAO Aircraft Engine Emissions Databank 28 . This formula is based on the weighted average of all possible engine types of the domestic routes in China.

Data availability
All the data on CCD emissions, LTO emissions, emission intensities is available as figshare datasets 32 .